1. Field of the Invention
The invention relates to a butterfly flap valve comprising a butterfly flap which for opening is adapted firstly to be axially displaced and then turned.
2. Description of Prior Art
A butterfly flap valve of the foregoing type, in which the shaft is journalled outside the flow cross-section is described in the U.S. Pat. No. 3,669,405.
Although this flap valve already has substantial advantages over previously known valves, it still has a few disadvantages. Owing to the long lever arm a high turning torque results. In the case of unfavourable manufacturing tolerances jamming of the links may result. Furthermore, a substantial number of pivoting joints is present in the medium to be controlled by the valve and these pivot points usually operate at temperatures substantially above room temperature and there is no possibility of lubricating them. Finally, owing to the three-point carrying arrangement also the flap valve is not carried or journalled in a statically defined manner.
It has been proposed to provide a further development of the flap valve in accordance with the U.S. Pat. No. 3,669,405 in which development the shaft runs within the flow cross-section in order to shorten the lever arm. However this construction still requires improvement, because the fabrication of the carrying arms arranged inside is complex and furthermore a large number of pivot or bearing points must be provided. Further, the bearing of the lever can practically only be made in two parts and comparatively high pressure losses are to be expected in he medium flowing through the valve owing to the interfering cross-sections of the parts extending into the flow.